1. Field of the Invention
The present invention relates to an optical disk drive and a method for driving the optical disk drive and more particularly relates to a focus control on an optical disk drive.
2. Description of the Related Art
In optical disk technologies, data or information can be read out from a rotating optical disk by irradiating the disk with a relatively weak light beam, and detecting the light that has been modulated by, and reflected from, the optical disk.
On a read-only optical disk, information is already stored as pits that are arranged spirally during the manufacturing process of the optical disk. On the other hand, on a rewritable optical disk, a recording material film, from/on which data can be read and written optically, is deposited by an evaporation process, for example, on the surface of a substrate on which tracks with spiral lands or grooves are arranged. In writing data on such a rewritable optical disk, data is written there by irradiating the optical disk with a light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the storage material film.
It should be noted that the depth of the pits, the depth of the tracks, and the thickness of the storage material film are all smaller than the thickness of the optical disk substrate. For that reason, those portions of the optical disk, where data is stored, define a two-dimensional plane, which is sometimes called an “information storage plane”. However, considering that such an “information storage plane” has a physical dimension in the depth direction, too, the term “information storage plane” will be replaced herein by another term “information storage layer”. Every optical disk has at least one such information storage layer. Optionally, a single information storage layer may actually include a plurality of layers such as a phase-changeable material layer and a reflective layer.
To read or write data from/on an optical disk, the light beam always needs to maintain a predetermined converging state on a target track on an information storage layer. For that purpose, a “focus control” and a “tracking control” are required. The “focus control” means controlling the position of an objective lens perpendicularly to the information storage layer (which direction will be referred to herein as a “substrate depth direction”) such that the focus position (or converging point) of the light beam is always located on the information storage layer. On the other hand, the “tracking control” means controlling the position of the objective lens along the radius of a given optical disk (which direction will be referred to herein as a “disk radial direction”) such that the light beam spot is always located right on a target track.
Various types of optical disks such as DVD-ROM, DVD-RAM, DVD-RW, DVD-R, DVD+RW and DVD+R have become more and more popular these days as storage media on which a huge amount of information can be stored at a high density. Optical disk drives compatible with those optical disks use an optical lens (i.e., an objective lens) with a numerical aperture (NA) of 0.6. Recently, however, in order to further increase the maximum densities and capacities of the optical disks, next-generation optical disks, including Blu-ray Disc (BD), have been under research and development and have already been put on the market. They suggest that an optical lens with an NA of at least 0.8 be used for such next-generation optical disks. A conventional optical disk drive of that type is disclosed in Japanese Patent Application Laid-Open Publication No. 11-273099 (see Paragraphs Nos. 9 through 18 and Nos. 35 through 51 and FIGS. 1 through 9 among other things), for example.
Japanese Patent Application Laid-Open Publication No. 11-273099 discloses that the level of a focus error (FE) signal changes with the intensity of a laser beam incident on an optical disk and also discloses a focus servo-control to be carried out using a “normalized FE signal”, which is obtained by dividing the FE signal by the intensity of reflected light. According to Japanese Patent Application Laid-Open Publication No. 11-273099, an electrical offset is produced in the normalized FE signal to a degree that changes with the mode of operation, which may be read, write or erase. This offset is caused by a circuit system for normalization and has mutually different magnitudes in the read, write and erase modes. Accordingly, even if this offset can be completely corrected in the read mode, for example, the offset may not be compensated for in the write and erase modes and the laser beam may be defocused.
Thus, in the focus servo control disclosed in Japanese Patent Application Laid-Open Publication No. 11-273099, the magnitude of an offset in the read mode is used as a reference offset, differences between the reference offset and an offset in each of the two other modes is calculated, and the offsets in the write and erase modes are compensated for appropriately, thereby minimizing the defocusing in all three modes of operation.
According to this conventional technique, the electrical offsets can be eliminated from circuit systems for the read, write and erase modes. However, this technique does not work fine if the read, write and erase modes have respectively different best focus positions.
The present inventors discovered as a result of researches that at a high NA, the read, write and erase modes had respectively different best focus positions. The reason will be described later. However, in the prior art, nobody has ever changed the focus position of a laser beam into its best position according to the mode of operation.
We also discovered that if the focus positions and the intensities of a laser beam were simply changed when the modes of operation were switched from read into write or from read into erase, then the focus control could not be carried out with high reliability.